This new application focuses on the determination of the mechanism by which the predominant extracelluar enamel matrix protein, amelogenin, regulates initial enamel mineral formation and tissue organization. Based on extensive prior findings, our overall hypothesis is that phosphorylation of serine-16 modulates the structure and hierarchical assembly of native (phosphorylated) full-length amelogenin, resulting in its greatly enhanced capacity to stabilize amorphous calcium phosphate (ACP) nanoparticles. We further hypothesize that subsequent proteolytic modification of native full-length amelogenin is required to promote the alignment and transformation of ACP to ordered bundles of apatitic crystals, as seen in the secretory stage of enamel development. We propose that enamelysin plays a critical role in this transformation process. The proposed studies are designed to provide fundamental insight into how matrix proteins, like amelogenin, control mineralization and structure in mineralized tissues. As a long-term goal, our findings should aid in the development of novel approaches for the regeneration and repair of diseased or damaged dental enamel. Given the high prevalence of dental caries, there is a tremendous need for restorative procedures that are superior to those presently available. Hypotheses will be tested in vitro, primarily using native amelogenins that contain a single phosphorylated site, and in vivo, using enamelysin null mice. Four specific aims will be carried out using multiple complementary approaches, including: dynamic light scattering, transmission electron microscopy, electron diffraction, Fourier-transform infrared spectroscopy, Raman microspectroscopy, small angle x-ray scattering, and cryomicroscopy. Specifically: Aim 1. To determine the effect of phosphorylation on the step-wise hierarchical assembly of native full-length amelogenin, to test the hypothesis that phosphorylation affects the formation and structure of amelogenin oligomers and their subsequent higher-order assembly; Aim 2. To determine the mechanism by which phosphorylated native full-length amelogenin effectively stabilize nanoparticles of ACP, to test the hypothesis that phosphorylation of the single serine-16 site affects protein conformation and structural changes that uniquely enhance the capacity of the full-length amelogenin to interact with forming ACP nanoparticles; Aim 3. To conduct in vitro studies to test the hypothesis that the alignment and subsequent transformation of ACP nanoparticles seen in vivo to ordered bundles of hydroxyapatite (HA) crystals is induced by specific proteolytic modifications of native full-length amelogenin; and Aim 4. To verify in vivo that enamelysin plays an essential role in regulating the transformation of aligned ACP particles to ordered bundles of HA crystals, by testing the hypothesis that disordered ACP-like minerals will persist in vivo, in the absence of enamelysin, in contrast to what is seen in wild type mice.